Although it seems contradictory, FishChannel.com states that in order to feel secure enough to swim in the open, goldfish like to have plenty of cover they can retreat to if needed. Goldfish may also hide if the current is too strong, agitating and stressing them, and making it difficult for them to swim.
First Tank Guide recommends that a tank be filled 50 to 75 percent with cover; goldfish are not particularly fussy about hiding places, so long as there are no sharp edges that may hurt them. Having sufficient cover helps minimize stress, improving their immune systems and reducing the chance of disease. First Tank Guide theorizes that this is because they are prey animals and are constantly wary of predators. FishChannel.com warns that fish can become so stressed that they stop eating and become vulnerable to parasites, their colors turning dull and washed out. Goldfish Connection warns that a current that is too strong may cause them to weaken, likening the experience to being forced to run on a treadmill all day; what appears to be hiding behavior is actually the goldfish avoiding the current. To determine if this is the problem, the owner of a goldfish can temporarily shut the filters off to see if the goldfish become more active.